Method of making a thermistor



3,359,632 METHOD OF MAKING A THERMISTOR John G. Froernel, Verona, and Meyer Sapoir, West Orange, N.J., assignors to Victory Engineering Corporation, Springfield, N.J., a corporation of New Jersey No Drawing. Filed Feb. 10, 1965, Ser. No. 431,695 14 Claims. (Cl. 29-620) ABSTRACT OF THE DISCLOSURE A method of making an electrical resistance element in which three distinct sizes of metal oxide materials difiering from each other by the order of 0.5 micron are mixed in a high hydroxyl binder to form a self supporting flake which is then cemented to a substrate by means of the binder within the flake.

This invention relates to a method for making electrical resistance elements. In particular, this invention relates to a method for making thin film resistance elements, especially those having a high temperature coefficient of resistance, known in the art as thermistors.

In the manufacture of thermistors, it is desirable to provide a structure with a short time response, that is, to design them in such a way that they are capable of being rapidly heated or cooled, and thus are quick to respond to temperature changes in electrical resistance. One way of doing this is to make them in the form of thin flakes.

Previous methods of making flake thermistors have in general proceeded along the lines of making a paste of the appropriate metal oxides in a suitable vehicle, spreading the paste on a flat surface, such as an optical flat, drying the paste, removing it from the flat, and firing it at an elevated temperature.

This method was subject to a number of disadvantages, including the necessity of handling thin, fragile flakes of material, and a tendency for the flakes to curl on firing. The tendency to curl is attributed to a gradient in the composition of the film of paste through the thickness thereof, caused by the tendency of the heavier particles to concentrate near the bottom.

In the attempt to overcome these difliculties, various other methods have been proposed. It has been proposed, for example, to minimize the abovementioned gradient by using a mixture of oxides ground to two different particle sides, so as to increase the packing fraction. Another method was to spread the paste directly into a cavity in the block to which it was to be applied, and firing it therewith, instead of first forming and firing the flake and then cementing it to the block. This method is successful in overcoming some of the above disadvantages, but involves difiiculties in control of the thickness and uniformity of the flake.

Still another method involves forming a more or less fluid paste of the metallic oxides in a thermoplastic binder, dropping it on the surface of a body of water inside a dam to limit the spread of the floating film, picking up the floating paste on a perforated ceramic wafer, and firing the paste on the wafer.

This method also possesses some advantages over the older methods, but is diflicult to control, because the thickness of the eventual flake depends on many factors, including the concentration of oxides in the paste, the viscosity of the vehicle, the size of the drop of paste placed on the water surface, and the area confined within the dam.

In those of the above methods which involve making a paste of the oxide mixture in a vehicle or binder, and then spreading the paste on a solid surface such as an optical flat or a grooved substrate block, some difficulty has been United States Patent experienced in laying down a film of uniform thickness and density. This has been largely due to the difficulty of finding a suitable vehicle or binder for use as the liquid portion of the paste. One method of applying the paste to the solid surface was to spray it on. This, however, required that the suspension be made using large amounts of vehicle, so that it might be described as a low-viscosity slurry, rather than a paste. The use of such a slurry, however, entailed the disadvantage that the vehicle was not sufliciently viscous to hold a dispersion of heavy metal oxides in a properly uniform suspension. The tendency therefore was for the heavy particles to settle to the bottom of the spray container. This made it extremely difficult to spray on a suspension that could be relied on to give a uniform thin flake of sintered oxide material after firing.

Another method of applying the suspension of oxides to a solid surface has been to use a doctor-blade, which is simply a straightedge provided with means for holding the edge at a predetermined distance above the underlying surface. This method overcame some of the difliculties inherent in the spray system, but was not a complete solution to the problem. If the vehicle was sufliciently viscous and gel-like in structure to maintain a uniform dispersion of oxide particles, it also tended to depart from the flow characteristics of a liquid, and fail to respond as expected to treatment with the doctor-blade. Such a vehicle, having a gel-like structure, has a tendency to pile up ahead of the blade and to exert a tension on those parts of the film that have already been passed over by the blade. The result of such tensions set up between one part of the film and another is that, as the tensions are equalized among themselves, some portions of the film tend to become thicker and others thinner. The resulting non-uniformity in film thickness, in turn, means that there is less oxide material in one part of the fired flake than another, leading to flakes of non-uniform thickness and unpredictable response characteristics. Also, flakes made from such a non-uniform film tend to curl and otherwise become deformed during the firing.

On the other hand, if the vehicle or hinder was formulated to be a relatively low-viscosity material, exhibiting less of the plastic flow of gel-like structure and more of the flow properties of a true liquid, the disadvantages were equally numerous. Using a suspension made from a vehicle having low viscosity and exhibiting liquid-type flow, there is no satisfactory way to avoid settling and segregation of the oxide particles, both before and after the film of oxide particle suspension is applied to the optical flat, ceramic substrate surface or the like. When such segregation occurred prior to the laying down of the film, the film laid down was of doubtful composition and uncertain uniformity. When it occurred after the film was formed, there was still a tendency for the heavier particles to migrate to the bottom of the film, causing a particle size gradient that resulted in development of internal stresses and a tendency for the flake to curl during firing. To date, no satisfactory solution to these difliculties has been proposed.

An object of the present invention is to provide an improved method for making electrical resistance elements.

Another object of the present invention is to provide a reliable method for making thermistors characterized by extreme uniformity of thickness and composition.

Still another object of the present invention is to pro vide a method which is easily carried out and in which the thickness of the resulting resistance element is easily controlled and reliably uniform.

A feature of the present invention is the use of a vehicle of controlled and critical chemical and physical properties, as more fully defined hereinafter.

Another feature of the present invention resides in the use of a suspension of metal oxide particles ground to at least three distinct size ranges.

Still another feature of the present invention is the use of a novel method of firing flakes of resistor material under conditions of uniform heating and mild mechanical restraint, as more fully described hereinafter.

Other objects, features, and advantages will become apparent from the following more complete description and claims.

The invention consists in the election and arrangement of ingredients and operating steps, as more fully described below. I

In one particularly desirable aspect, this invention contemplates a method of making an electrical resistance element which comprises in combination the steps of providing a fluid dispersion of a three-size metal oxide material in a liquid, high-hydroxyl binder vehicle, depositing said fluid dispersion on a flat surface, leveling said deposited dispersion with a doctor-blade, drying the leveled dispersion, and firing the dried dispersion at a temperature at least equal to the sintering temperature of said metal oxide material.

In another embodiment, the invention contemplates a metal oxide composition having novel physical characteristics which impart particularly dense packing qualities.

In still another embodiment, the invention contemplates a binder vehicle useful in the preparation of flake thermistors and the like, said vehicle comprising in combination a polyvinyl butyral resin having a hydroxy content of the order of 20% by weight of said resin and an acetate content of the order of 2.5% by weight of said resin, and an average molecular length not exceeding about 500 Angstroms; a polyvinyl alcohol having an average molecular length not exceeding about 500 Angstroms; a lower aliphatic alcohol; a lower alkyl ester of a lower aliphatic acid, and an aromatic hydrocarbon solvent.

The metal oxide material may be any oxide or mixture of oxides which, upon firing, sinter together to form a cohesive body useful as an electrical resistance element. In place of the oxides themselves, one may use other compounds that decompose to yield the oxides at sintering temperatures, and the term metal-oxide-material, or

the like, as used herein, should be understood to include such oxide yielding compounds. With particular respect to thermistors, conventional materials are well known in the art and need not be described in detail. Among such conventional materials are the triple oxide compositions.

A preferred example of such a composition is the following: Percent by weight 7 of metals and oxide Manganese oxide, Mn O 56.0 Nickel oxide, NiO 14.0 Cobalt oxide, C 0 30.0

The oxides used as starting materials are preferably in powdered form, as this reduces the amount of grinding needed to prepare them for use in the present invention. Whatever the fineness of the oxides as supplied, they are ground to a degree such that the average particle diameter is about 2.0 microns or less, preferably about 1.8 microns.

particle diameter, the metal oxide material is divided into three portions. The first portion is further milled down to an average particle diameter of 1.5 microns or less, the second to an average particle diameter of 1.0 micron or less, and the third to an average particle diameter of 0.5 micron or less.

Approximately equal amounts by Weight of the three size-range fractions thus produced are taken as the metal oxide material component of the dispersion.

At least three distinct size-range fractions are needed to achieve a satisfactory packing fraction. Two size-range fractions are an improvement over a single uniformly ground material, but do not give films dense enough to be satisfactory for purposes of this invention.

The binder vehicle employed in the preferred practice of the invention comprises a polyvinyl butyral resin having a hydroxyl content of the order of 20% by weight, an acetate content of the order of 2.5% by weight, and having an average molecular length not exceeding about 500 Angstroms. Any polyvinyl butyral resin satisfying these criteria will function satisfactorily in the practice of the invention. One resin that has proven particularly satisfactory in practice is that known as Butvar resin, and marketed by the Shawinigan Chemical Co., as Shawinigan Resin B-72A.

Another component of the binder vehicle is polyvinyl alcohol. The function of this component is primarily to augment the hydroxyl content of the non-volatile portion of the vehicle. The polyvinyl alcohol should also have an average molecular length not in excess of about 500 Angstroms.

The volatile portion of the binder vehicle comprises a lower aliphatic alcohol, a lower alkyl ester of a lower aliphatic acid, and anaromatic solvent. In the preferred embodiment, these are methyl alcohol, amyl acetate and toluene, respectively.

As to the relative proportions of the various components of the binder vehicle, it is preferable that the resin, or non-volatile, portion of the vehicle constitute about 50% of the weight thereof, with the volatile components accounting for the other 50%. The preferred composition of the non-volatile portion is about three parts of polyvinyl butyral for each two parts (by weight) of polyvinyl alcohol.

The proportions of the components in the volatile portion of the vehicle may vary considerably, but as an illustration, a successful vehicle has a volatile portion made up in proportions of six parts by weight of methyl alcohol, one part of amyl acetate and three parts of toluene.

The oxide material, ground as above described, is blended into the binder vehicle. Approximately equal amounts by weight of oxide material are taken from each of the size-range fractions, and the oxide material is mixed into approximately twice its total weight of the binder vehicle. If necessary, the resulting paste may be cut with a thinning solvent consisting of a mixture of 20% amyl acetate and methyl alcohol.

The fluid suspension thus prepared is deposited on a flat surface, for example a glass optical flat, and doctorbladed to the desired film thickness. The film is cut to the desired shape and the excess portions removed from the surface of the flat. The remaining blank is wetted by means of a paper towel applied over the film. This is then allowed to dry.

The flakes are then removed from the flat and placed in an ignition boat lined with refractory powder such as 200 mesh zirconia powder and the flakes are covered with the same refractory powder. The whole is then fired for an appropriate time and at a suitable temperature to sinter the particles of oxide material. The refractory powder insures uniform heating of all parts of the flake and also provides a slight mechanical restraint against changes in shape. Both of these effects are helpful in overcoming any residual tendency for the flakes to be distorted during firing.

After the flakes have been fired, they are allowed to cool and removed from the boats. Preferably, they should then be cleaned, for example, using methyl alcohol in an ultrasonic cleaner.

The final stage in preparation of the resistance element is to apply suitable contacts or electrodes, to which electrical leads may subsequently be attached. This may be done in a number of ways, as will be obvious to those skilled in the art. The preferred method is vacuum evaporation, and the material of the electrodes may be a metal such as aluminum, gold, silver, platinum, and other metals and alloys well known in the art.

In order to illustrate more fully the nature of this invention and the manner of practicing the same, the following example is presented:

Example A mixture is made from the powdered oxides of manga nese, nickel and cobalt in the following proportions:

Percent by weight of metals and oxide Manganese oxide, Mn O 56.0 Nickel oxide, NiO 14.0 Cobalt oxide, C 0 30.0

The mixture is ground in a wear-resistant ball mill using aluminia balls, until the particles have an average cross-sectional diameter of 1.8 microns.

The ground mixture is then divided into three portions,

and the three portions are ball milled separately as follows:

Portion N 0. Hrs. milled Aug. Diameter,

microns A binder vehicle is prepared by mixing the following ingredients:

Percent by weight Butvar resin (Shawinigan Resin B-72-A) 30 Polyvinyl alcohol 20 Methyl alcohol Amyl acetate 5 Toluol 15 Percent by weight Oxides, portion No. 1

Oxides, portion No. 2 10 Oxides, portion No. 3 10 Binder vehicle 60 Thinning solvent 10 The resulting dispersion is ball-milled for 24 hours. After ball-milling, the dispersion is deposited on a glass flat, doctor-bladed to the desired thickness, and trimmed to shape. A moistened paper towel is laid on the resulting blank and allowed to dry.

and by the use of methyl alcohol, cement the green flake to substrates such as alumina, BeO, nickel, silicon, lava, barium titanate, etc. The cement is formed via the methyl alcohol attacking the binder to a minor extent causing an intimate contact between the green flake and the substrate.

These in turn are fired at temperatures between 1100 C. and 1300 C., depending upon the substrate to obtain a cement-free intimate bond between the sintered flake and the substrate. This bond is enhanced by an increase in the porosity of the substrate.

The substrate to which the thermistor flake is applied may be a ceramic material or a metal. Many materials of both classes are well known in the art. Typical ceramic substrates are BeO, MgO, A1 0 boron nitride and quartz. Ceramic or metal materials may be used which are unreactive with the thermistor oxides materials and capable of withstanding the firing temperatures used to cure them.

A typical metallic substrate is nickel.

While this invention has been described with reference to certain preferred embodiments and illustrated by way of certain examples, these are illustrative only, as many alternatives and equivalents will readily occur to those skilled in the art, without departing from the spirit and proper scope of the invention. The invention is therefore not to be construed as limited except as set forth in the appended claims.

Having thus fully described the invention, what is claimed as new and desired to be secured by Letters Patent of the United States, is:

1. A method of making an electrical resistance element which comprises in combination the steps of providing a metal oxide material in the form of at least three distinct size-range fractions diifering from each other in size by the order of 0.5 micron, the largest of said three sizerange fractions having an average particle diameter not exceeding about 1.5 microns, forming a fluid dispersion of said metal oxide material in a liquid, high-hydroxyl binder vehicle, depositing said fluid dispersion on a flat surface, leveling said deposited dispersion with a doctorblade, drying said leveled dispersion, and firing the dried dispersion at a temperature at least equal to the sintering temperature of said metal oxide material.

2. A method of making an electrical resistance element which comprises in combination the steps of providing a metal oxide material in the form of at least three distinct size-range fractions differing from each other in size by the order of 0.5 micron, the largest of said three sizerange fractions having an average particle diameter not exceeding about 1.5 microns, forming a fluid dispersion of said metal oxide material in a liquid, high-hydroxyl binder vehicle, depositing said fluid dispersion on a flat surface, leveling said deposited dispersion with a doctorblade, drying said leveled dispersion, removing said dried dispersion from said flat surface as a free, self-supporting green flake, embedding said flake in a bed of refractory material, and firing said flake at a temperature at least equal to the sintering temperature of said metal oxide material.

3. A method of making an electrical resistance element which comprises in combination the steps of providing a metal oxide material in the form of at least three distinct size-range fractions differing from each other in size by the order of 0.5 micron, the largest of said three sizerange fractions having an average particle diameter not exceeding about 1.5 microns, forming a fluid dispersion of said metal oxide material in a liquid, high-hydroxyl binder vehicle, depositing said fiuid dispersion on a flat surface, leveling said deposited dispersion with a doctorblade, drying said leveled dispersion, removing said dried dispersion from said fiat surface as a free, self-supporting flake, depositing said flake on a ceramic substrate, moistening said flake and said substrate with a volatile solvent in which said binder vehicle is at least partially soluble, and firing said flake and said substrate at a temperature at least equal to the sintering temperature of said metal oxide material.

4. A mehtod according to claim 3, wherein said volatile solvent is a lower aliphatic alcohol.

5. A method acording to claim 4, wherein said lower aliphatic alcohol is methyl alcohol.

6. A method according to claim 3, wherein said substrate consists essentially of alumina.

7. A method according to claim 3, wherein said substrate consists essentially of beryllium oxide.

8. A method according to claim 3, wherein said substrate consists essentially of silica.

9. A method according to claim 3, wherein said substrate consists essentially of barium titanate.

10. A method according to claim 3, wherein said substrate consists essentially of boron nitride.

11. A method according to claim 3 wherein said substrate is a metal.

12. A method according to claim 3 wherein said substrate is nickel.

13. A method according to claim 3 wherein said substrate is silicon.

14. A method according to claim 3 wherein said substrate is lava.

References Cited UNITED STATES PATENTS 2,633,521 3/1953 Becker et al. 2,674,583 4/1954 Christensen. 3,019,198 1/1962 Dumesnil 33822 3,052,573 9/1962 Dumesnil 338-308 OTHER REFERENCES Kingery: Introduction to Ceramics, 1961, pp. 33- 35.

Van Vlack: Elements of Materials Science, 1960, pp. 290299.

CHARLIE T. MOON, Primary Examiner.

J. L. CLINE, Assistant Examiner. 

1. A METHOD OF MAKING AN ELECTRICAL RISTANCE ELEMENT WHICH COMPRISES IN COMBINATION THE STEPS OF PROVIDING A METAL OXIDE MATERIAL IN THE FORM OF AT LEAST THREE DISTINCT SIZE-RANGE FRACTIONS DIFFERING FROM EACH OTHER IN SIZE BY THE ORDER OF 0.5 MICRON, THE LARGEST OF SAID THREE SIZERANGE FRACTIONS HAVING AN AVERAGE PARTICLE DIAMETER NOT EXCEEDING ABOUT 1.5 MICRONS, FORMING A FLUID DISPERSION OF SAID METAL OXIDE MATERIAL IN A LIQUID, HIGH-HYDROXYL BINDER VEHICLE, DEPOSITING SAID FLUID DISPERSION ON A FLAT SURFACE, LEVELING SAID DEPOSITED DISPERSION WITH A DOCTORBLADE, DRYING SAID LEVELED DISPERSION, AND FIRING THE DRIED DISPERSION AT A TEMPERATURE AT LEAST EQUAL TO THE SINTERING TEMPERATURE OF SAID METAL OXIDE MATERIAL. 